O.K. I just looked at the Faichild offerings in 300-500V devices. I only found 1 with the transcunductance like we might be after. The others were 3 or 4 times higher? There were some things like thermal resistance which in some cases were unbeleiveably low. Probably a big die! and with that will be big capacitance! About double ussually!
As I was saying, the 16N25C is a great choice

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"It was the perfect high end audio product: Exotic, inefficient, expensive, unavailable, and toxic." N.P.

FLG,
Have you read the link above about the discontinuation of the original parts, specifically the refernce ti the P chaanel performance. I assume his statement can be taken in context of using the 19n20c complimentary part. If this is a safe assumption, it seems safest to try to match the performance of those parts. To look for unique replacements opens up too many variables in this case. I have some of the newer parts, which happen to have higher yfs, with the transfer curv epushed over in such a way as to test charles' theory. I will post measurements on all, but point for point, i think the 16n part is the closest we may come to the original in performance when viewed within the context of th eleast amoun to fpossible changes. I am going to read the Hansen thread again, as he was a lateral fan, as am I. The IRF9240 hump may or may not be present with this Fairchild part.

I will join in and say
Realy great that you are taking the time with this.

And that is the main thing
I have noticed the pool and will abstain from voting as I have been using FQP19N20 with FQA12P20 since last August
As I have made no comparrison, sound wise, with other devices I can not express any constructive opinion.
Still To me it does sound like one of the best amplifier I ever Had.

IMO FQA19N20L would be baad as it may match worse with Positive counterpart.

If I may.. Why not ask Papa for help?
I am shure that once he is avvare of your (most laudable) effort he may chip in whit same advice.

Papa has been asked, but I would not be surprised if he doesn't answer. He wants us greedy boy'z to figure somethings out. Hopefully there will be some insight, but i think it is possible that we can over-think this. Without some anomaly in the response like in the thread mentioned by FLG concerning IRF P channel parts, we should get pretty decent performance from this part based on what we see in the datasheet. I think it may be possible to offer FQP alternatives for those who desire them, but I would like to stay with larger form devices for sake of heat and ease of use. Especially with diode version of F5T, we are dealing with some settings that are going to be sensitive to heat level and having bigger and more easily cooled parts may help somewhat. That being said, BKsabath is proof the other works. Kudos to him for branching out with the different part.

Buzzforb, it is possible that Charles's comments were 5 years ago before the introduction of some of the parts we are seeing today? It didn't really look like his enference was true. But, what he was emphasising seemed to be transconductance P-N matching? Right?
We don't have that in any pair we play with except the 1530/201. We seem to be focused on matching capacitance or something otherwise?

__________________
"It was the perfect high end audio product: Exotic, inefficient, expensive, unavailable, and toxic." N.P.

I am almost 100% sure that i have secured what i believe to be the best alternative to the 19n20C. I have one more lead on the original, but if it doesn't pan out, I would suggest FQA16n25C. I will provide links to both datasheets and I believe you will see that they are almost identical in performance.

Papa has been asked, but I would not be surprised if he doesn't answer. He wants us greedy boy'z to figure somethings out. Hopefully there will be some insight, but i think it is possible that we can over-think this. Without some anomaly in the response like in the thread mentioned by FLG concerning IRF P channel parts, we should get pretty decent performance from this part based on what we see in the datasheet. I think it may be possible to offer FQP alternatives for those who desire them, but I would like to stay with larger form devices for sake of heat and ease of use. Especially with diode version of F5T, we are dealing with some settings that are going to be sensitive to heat level and having bigger and more easily cooled parts may help somewhat. That being said, BKsabath is proof the other works. Kudos to him for branching out with the different part.

One thing I noticed with the comparison of the 19n20c part with the 19n25c part was the difference in the drain-source on resistance, Rds(on). 0.14 Ohms vs 0.22 Ohms respectively.

Has anyone done any measurements or modelling of this, or even have any thoughts? The P-channel 12p20 has about 0.36 Ohms on resistance, so maybe you are right buzzforb and the 19n25c is a better match.